Most living organisms require iron for their growth and reproduction. Plants take in solubilized iron existing in soils, thereby constituting a major iron inflow into animals and humans. Although mineral iron is abundant in soils, such iron is sparingly soluble under aerobic conditions at high pH. Consequently, in alkali soils which account for about 30% of the world's cultivated soils, plants often exhibit iron deficiency symptoms called “chlorosis” (yellowing from chlorophyll imperfection), reducing crop yield and quality. Accordingly, plants exhibiting high tolerance to the iron deficiency have been strongly desired.
In order to take in iron, higher plants employ iron-uptake strategies under iron-deficient conditions, that is, chelation (Strategy II) and reduction (Strategy I). Among these, Strategy II is specific to graminaceous plants. Strategy II is such that (i) iron in soils is chelated by use of the muginetic acid family phytosiderophores (MAs) which are natural iron chelators, and (ii) the iron thus chelated is taken into cells via transporters (YS1) existing in cell membranes of cells in roots.
To date, extensive physiological, biochemical and molecular studies identified a biosynthetic pathway of MAs and genes encoding biosynthetic enzymes (such as nicotianamine synthase (NAS), nicotianamine aminotransferase (NAAT), IDS2, and IDS3). There has been known that expression of these biosynthetic enzymes is coordinately increased in response to iron deficiency.
However, molecular mechanisms for increasing the expression of genes related to the response to the iron deficiency, that is, genes related to iron uptake under the iron-deficient conditions, are poorly understood. The inventors of the present invention analyzed a promoter region of a barley IDS2 gene which is induced to express under the iron-deficient conditions, thereby identifying novel iron-deficiency-responsive cis-elements IDE1 (Fe deficiency responsive element 1) and IDE2 (Fe deficiency responsive element 2), which synergistically induce, in tobacco roots as well as in rice roots and leaves, expression of the genes related to the response to the iron deficiency (Patent Literature 1).